3GPP LTE (3rd Generation Partnership Project Long-term Evolution) adopts a ZC (Zadoff-Chu) sequence as a signal sequence used for a reference signal for data demodulation (DMRS: DeModulation-Reference Signal) used on an uplink. The ZC sequence is used when the transmission bandwidth is 3 RBs (resource blocks) or more.
On an LTE uplink, many DMRS sequences are divided into 30 sequence groups in each transmission bandwidth (1 to 110 RBs). In each sequence group, as shown in FIG. 1, a transmission bandwidth (more specifically, the number of RBs allocated) and a DMRS sequence are associated with each other. Respective sequence groups are assigned different numbers (sequence group number u=0 to 29), and as shown in FIG. 2, each cell is assigned one sequence group from among #0 to 29. Such sequence group assignment is called “cell-specific sequence group assignment” or “cell-specific assignment.” A base station (which may also be called “eNB”) broadcasts cell IDs to terminals (which may also be called “UE (User Equipment)”) in the cell. Since the cell IDs and 30 sequence group numbers are uniquely assigned with each other beforehand, terminals in the cell can know cell-specific sequence group numbers from the broadcast cell-specific IDs. Even when the transmission bandwidth is changed, the terminal can identify a DMRS sequence number from only a sequence group number. Thus, the cell-specific assignment can reduce signaling of sequence numbers. In the cell-specific assignment, different sequence groups are assigned to nearby cells in order to reduce inter-cell interference.
The ZC sequence is a kind of CAZAC (Constant Amplitude and Zero Auto-correlation Code) sequence and is expressed by following equation 1.
                    (                  Equation          ⁢                                          ⁢          1                )                                                                                                x              q                        ⁡                          (              m              )                                =                      e                                          -                j                            ⁢                                                π                  ⁢                                                                          ⁢                                      qm                    ⁡                                          (                                              m                        +                        1                                            )                                                                                        N                  ZC                  RS                                                                    ,                  0          ≤          m          ≤                                    N              ZC              RS                        -            1                                              [        1        ]            
In equation 1, NZCRS is a sequence length of a ZC sequence, q is a ZC sequence number, and m is an element number of the ZC sequence. Sequence length NZCRS is a maximum prime number that does not exceed the number of subcarriers in a transmission bandwidth of DMRS, and (NZCRS−1) ZC sequences having good cross-correlation characteristics can be generated. ZC sequence number q is calculated by equation 2.[2]q=└q+½┘+v·(−1)└2q┘q=NZCRS·(u+1)/31  (Equation 2)
Here, regarding v, v=0 when the transmission bandwidth is 5 RBs or less and v=1 when the transmission bandwidth is 6 RBs or more. Sequence group number u is an integer of u=0 to 29. Number u is associated with a cell ID of a serving cell and all UEs in the cell each use one of sequences that belong to a common sequence group.
Equation 2 means calculating a ZC sequence number (when v=0) corresponding to q/NZCRS, which is a ratio between a ZC sequence number and a ZC sequence length closest to “(u+1)/31,” and a ZC sequence number (when v=1) corresponding to q/NZCRS second closest to “(u+1)/31.” Thus, as ZC sequences of each RB, a plurality of sequences having close q/NZCRS values are assigned to the same sequence group. In the following description, a value that serves as a reference for calculating a ZC sequence number such as “(u+1)/31” of equation 2 is called “sequence selection reference value.” Here, 31 is a maximum prime number (minimum ZC sequence length) that does not exceed the number of subcarriers (=36) assigned to minimum RBs (=3 RBs) of the sequence group. Thus, the sequence selection reference value means a ratio between a sequence group number and a minimum ZC sequence length. Furthermore, a ratio between a ZC sequence number that determines a ZC sequence and the ZC sequence length, that is, “q/NZCRS” is called a “sequence determination value.”
Sequences of close ZC sequence q/NZCRS have a feature of having similar waveforms and a high cross-correlation between sequences. Thus, a sequence group used in one cell is configured of a combination of ZC sequences having a high cross-correlation between ZC sequences, and the probability that sequences having a high cross-correlation may be used in neighboring cells is thereby reduced, making it possible to reduce interference between neighboring cells (e.g., see PTL 1).
DMRS used in 3GPP LTE is transmitted with a transmission bandwidth which is an integer multiple of 1 RB consisting of 12 subcarriers. Thus, the sequence length of a ZC sequence which is a prime number does not coincide with the number of subcarriers corresponding to a transmission bandwidth of DMRS. Thus, as shown in FIG. 3, a sequence obtained by copying (called “extension”) the top portion of a ZC sequence having a prime number sequence length to the end portion is used as DMRS to be actually transmitted. For example, as DMRS to be transmitted in 3 RBs (36 subcarriers), a ZC sequence having sequence length NZCRS of 31 is used and DMRS is generated by copying the first five elements to the end portion.
In LTE-Advanced, which is an evolved version of LTE, a heterogeneous network (HetNet) using a plurality of base stations providing coverage areas in different sizes is under study to achieve a further capacity improvement. In the operation of HetNet, a pico cell having low transmission power is deployed within a coverage area of a macro cell having high transmission power. The macro cell may also be called “HPN (High Power Node).” The pico cell may also be called “LPN (Low Power Node)” or “low power RRH (Remote Radio Head).”
In LTE-Advanced, application of CoMP (Coordinated multiple point transmission and reception) is also under study, which is a communication scheme in which a plurality of cells (base stations) cooperate to transmit and receive signals to and from a terminal in a HetNet environment. CoMP is mainly intended to improve the throughput of a user located at a cell edge. For example, in the case of uplink CoMP (UL_CoMP), a plurality of cells (which may also be called “base stations” or “reception points”) cooperate to receive uplink signals from one terminal, and received signals are combined by a plurality of cells to improve receiving quality.
In UL_CoMP, the introduction of MU-MIMO (Multiple User-Multiple Input Multiple Output) communication is under study to achieve a further system performance improvement effect. MIMO communication is a technique in which transmitting and receiving sides are provided with a plurality of antennas to enable different signal sequences to be simultaneously and spatially multiplexed at the same frequency. MU-MIMO communication is a technique in which MIMO communication is carried out by a plurality of terminals to which UL_CoMP is applied, that is, terminals that cooperate to receive and combine transmission signals in a plurality of cells (hereinafter, may also be referred to as “CoMP_UE”) and a base station. MU-MIMO communication can improve the frequency utilization efficiency of the system.
In MU-MIMO communication, it is necessary to transmit DMRSs which are orthogonalized among terminals to demultiplex signals of different terminals. As a method of orthogonalizing DMRSs, a ZC sequence (CS-ZC sequence) may be used in which a different cyclic shift (CS) for each terminal is applied. Setting a value larger than a maximum propagation delay time of transmission signals of terminals as a cyclic shift value makes it possible to orthogonalize a plurality of CS-ZC sequences generated from ZC sequences of the same sequence group.
However, when UL_CoMP is applied, DMRSs need to be received from a plurality of different cells. For this reason, the aforementioned cell-specific sequence group assignment may cause ZC sequence numbers of CoMP_UE to differ from each other, making it impossible to orthogonalize CS-ZC sequences to be used as DMRSs.
Thus, as shown in FIG. 4, studies are being carried out on the possibility of introducing UE-specific sequences (sequence group #17 in FIG. 4) in which sequences are indicated individually for the respective terminals instead of cell-specific sequences (sequence groups #1 and #2 in FIG. 4) for CoMP_UE. Assigning the same sequence group to terminals that perform CoMP reception allows DMRSs to be orthogonalized among CoMP_UEs.